In a conventional wireless communication system constituted by a plurality of base stations, a technique referred to as handover is used (For example, see Patent Document 1). In a conventional handover method, a mobile station measures received powers of signals received from the plurality of base stations. On the basis of the measurement results, a first base station providing a maximum received-power to the mobile station and a second base station providing the second highest received-power to the mobile station are selected. A condition is used as a trigger which causes the mobile station to switch the base station. The condition is that a received-power difference between the first and second base stations is smaller than a preset threshold value and time at which the difference is smaller than the threshold value is equal to or longer than set time set in advance. As a result, the mobile station can maintain high communication quality by switching between the base stations with reference to the received-power difference. Furthermore, only when the time at which the received power difference is smaller than the threshold value is equal to or longer than the set time, the mobile station switches between the base stations. Therefore, a system load caused by switching between the base stations can be reduced.
A conventional handover process will be described below with reference to the flow chart shown in FIG. 1. First, forward (downward) pilot signals are received from first to Nth base stations from which a mobile station receives signals, forward propagation quality is measured (step S101). Next, a power difference between the first base station having the maximum received-power and the base station having the second highest received-power is calculated (step S102).
When the received-power difference between the first and second base stations is smaller than the threshold value set in advance (step S103), and it is determined that time at which the difference is smaller than the threshold value is equal to or longer than the set time set in advance (step S104), switching between the base stations is carried out (step S105).
On the other hand, a wireless transmission method which requires synchronization between users of base stations includes a method which performs synchronization by using Cyclic Prefixes to control a timing error between the users within a Cyclic Prefix so as to prevent interference between the users (For example, see Non-patent Document 1).    Patent Document 1: Japanese Patent Application National Publication (Laid-Open) No. 2003-500909    Non-patent Document 1: Variable Spreading and Chip Repetition Factors (VSCRF)-CDMA in Reverse Link for Broadband Packet Wireless Access, Y Goto, T Kawamura, H Atarashi, M Sawahashi, IEICE Commun., VOL. E88-B, NO. 2 Feb. 2005